The fact that Windows claims it's unable to find sfc.exe could be due to a faulty or missing path statement in the settings on your system.
(You can check this in System Properties/Advanced/Enviroment settings)
However, it should work ok if you put the whole path in the run-command.
But as you say yourself, you might have a problem if running XP SP2.
sfc.exe should still be able to run, but you will need either both the original XP CD, as well as SP2 on a CD, as sfc will ask for the original disk(s) if it finds files which needs to be changed/updated.
The best would be to have/make a so called slipstreamed XP SP2 CD, which will include all the correct/updated file versions.
Autostreamer is a good freeware option for doing just that (you will need the original XP CD and SP on a CD), and you can download it here:
http://fileforum.betanews.com/detail/1092632287/1
By the way, the same goes for running a repair install of XP.